Hey there -
Just joined the forum. Hoping for some insight/help.
For almost 1.5 years, I've been having these symptoms:
- When the engine is cold (after starting), the RPMs will drop and the car will stall, unless I rev the engine up. This usually happens when backing out of a parking spot or at the first stop sign/light I approach as I hit the brakes.
- When trying to restart the engine, it is slow to turn over and start. Usually the rpms will drop again and it will stall.
- Once the engine heats up to regular driving temps, it will not stall.
- Around 1400 rpm, the front end of the vehicle trembles. Any acceleration or deceleration (just letting off the gas) will stop the symptoms.
- Poor fuel economy. When the car was behaving correctly, I could get around 400 miles to a tank (highway) or 350+ around town miles to a tank. Now I get around 225 to a tank.
Code reader shows random misfiring history and active codes. I've had the throttle body cleaned out. Mechanic said it might be the catalytic converter, but I have excellent acceleration and no loss of power.

Luckily, I have a great friend that is a former master tech/mechanic. UNluckily he no longer has his shop, and his gear is all down in Atlanta (we live in Alaska). He believes (after test driving and checking the vehicle out as much as he can) that the mis-firing may be due to a faulty coolant temp sensor (for the computer, NOT the gauge).
I've already tossed nearly $2,700 at this problem over the past 2 years, and I hate to keep throwing more money/parts at it. Wondering if anyone here has had similar issues and found a fix? Need help...
Thanks!

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Do you smell fuel ? It may need spark plugs . Mine.did something similar . My gas mileage dropped and I could.smell.fuel .

No gas smell, and the plugs were changed last year... and then again last week... for which I was charged $27 PER plug (they were $9 plugs, I checked NAPAs prices - and this service place gets their parts FROM NAPA) + $70 freight to get them shipped to Alaska, because the service writer said no one had the special plugs in stock in the state. Only they DID at EVERY DAMN NAPA LOCATION. Sorry, a bit frustrated

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Vibration around 1400 rpm could be worn engine/transmission mounts ... there's one engine mount on right-side and front and rear transmission mounts near center-left of power-train.

I'd check electrical system to make sure battery and generator are performing as expected.

Its a good idea to test fuel pressure and always try to use the best gas you can afford to keep engine clean.

There's lots more thoughts and I'll try to put those in private message.

Thanks. I don't have the codes, unfortunately.
The car has 135,000 on it, but this started around 115-120,000 miles (roughly). Car has had regular service, although I don't believe the timing belt has been done (I could be wrong though...)
The rumble at 1400 is definitely due to a misfire, this has been verified by the code reader. In Alaska, we have more additives in our fuel than most places, but I buy from reputable stations whenever possible. Battery tests out fine, as does the generator.

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Thanks. I don't have the codes, unfortunately.
The car has 135,000 on it, but this started around 115-120,000 miles (roughly). Car has had regular service, although I don't believe the timing belt has been done (I could be wrong though...)
The rumble at 1400 is definitely due to a misfire, this has been verified by the code reader. In Alaska, we have more additives in our fuel than most places, but I buy from reputable stations whenever possible. Battery tests out fine, as does the generator.

You mentioned in one post the engine was slow to crank after engine died. I read this as low battery voltage and suggested you check electrical system. You confirmed the system was good but I know of four reasons for a cold engine to crank slowly: (1) low battery power, (2) bad electrical cables (plus and/or minus), (3) failing starter, or (4) advanced engine timing.

Regarding codes - this is how to save lots of money. Most any parts store will read codes for you (although AutoZone code readers aren't typically compatible with Vue data-bus). Most malfunctioning sensors (ECT, HO2S, etc) should set a code and can keep you from wasting money on unneeded parts.

Do you see flashing MIL (malfunction indicator light) in 1400 range (rumble)? This is a good indication of misfire.

Next time you notice rumble put gear selector to "I" and speed up or slow down for 1400 on tachometer - and check to see if rumble is still present. The noise might well be torque converter shudder ... a well-known issue caused by dirty ATF.

I suspect if the "rumble" is severe enough it could cause the engine knock-sensors and PCM to greatly retard timing which will skew engine performance and possibly any monitoring test equipment.

One possible cause of poor low RPM operation after cold start is idle-learn procedure was not completed after PCM programming or throttle body cleaning. See attached procedures (apply to any 3.5)

Idle-learn steps are related to engine coolant temp. You can guesstimate engine temp using the temp gauge - once the needle is slightly past the 1/4 mark it'll stabilize in that area as the thermostat cycles to maintain about 180°F.

When needle moves to slightly left of mid-range mark - coolant temp is near 210°F (cooling fans will run low-speed at 210).

You mentioned in one post the engine was slow to crank after engine died. I read this as low battery voltage and suggested you check electrical system. You confirmed the system was good but I know of four reasons for a cold engine to crank slowly: (1) low battery power, (2) bad electrical cables (plus and/or minus), (3) failing starter, or (4) advanced engine timing.

Regarding codes - this is how to save lots of money. Most any parts store will read codes for you (although AutoZone code readers aren't typically compatible with Vue data-bus). Most malfunctioning sensors (ECT, HO2S, etc) should set a code and can keep you from wasting money on unneeded parts.

Do you see flashing MIL (malfunction indicator light) in 1400 range (rumble)? This is a good indication of misfire.

Next time you notice rumble put gear selector to "I" and speed up or slow down for 1400 on tachometer - and check to see if rumble is still present. The noise might well be torque converter shudder ... a well-known issue caused by dirty ATF.

I suspect if the "rumble" is severe enough it could cause the engine knock-sensors and PCM to greatly retard timing which will skew engine performance and possibly any monitoring test equipment.

One possible cause of poor low RPM operation after cold start is idle-learn procedure was not completed after PCM programming or throttle body cleaning. See attached procedures (apply to any 3.5)

Idle-learn steps are related to engine coolant temp. You can guesstimate engine temp using the temp gauge - once the needle is slightly past the 1/4 mark it'll stabilize in that area as the thermostat cycles to maintain about 180°F.

When needle moves to slightly left of mid-range mark - coolant temp is near 210°F (cooling fans will run low-speed at 210).

I'm going to have a mechanic friend with a scanner run the scan early this week.
I haven't had any flashing indicator lights, just a SES light. We had the codes read last weekend (at an AutoZone), and it came up with multiple misfires, both stored and active codes (although, I don't have the codes anymore). I tried the "I" driving, as you suggested. But it is very difficult to get the RPMs to stick around 1400 rpm. I've only been able to consistently make the 'rumble' happen when in Drive, at about 45 mph and 1400 RPMs - and then when using cruise control OR maintaining a steady speed manually AND when on a flat road. I've also noticed that it seems to shift rough - even over very small dips/rises in the road. It will quickly downshift when the road dips, and quickly ramp up the rpms when coming up the other side. Not sure if that is related, but I thought I'd mention it. I'm going to check the Transmission Fluid next to see if it looks good.
We will also try the idle-learn procedure when we scan it. This is interesting... About 3 years ago, I got a letter from Saturn regarding a voluntary/recommended service that involved (I believe...) just a reprogramming of the computer (yeah, right now I really wish I had paid better attention to what maintenance was being done. I feel like an idiot...). So, I took the car into Anchorage - to the only Saturn authorized maintenance shop in Alaska - a GM dealer. They did the recommended (free) reprogramming, and I picked the car up afterwards. I started the car in their parking lot and pulled up to the stop sign - where the car promptly stalled. This was the first time it ever stalled. Of course, I looked in the rearview mirror at the service center to see the 'Open' sign turn off... So, yeah - that's where the whole thing started. I did take the car back the next day, but they couldn't find anything wrong except that they recommended that the cabin air filter needed to be changed... riiiiiiight...
Regarding my engine temp, the gauge (once it stabilizes after about 5-10 minutes of driving) ends up around 1/3rd of the range, about half-way between the 1/4 and 1/2 mark temp marks.
Thanks again, for all of your help.

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Ok - so a quick update. Had my friend use his pro scanning tool and it came back with some odd air intake temp sensor readings. Sensor #2 read 54 degrees (correct), but sensor #1 read 200+ - this is when the vehicle/engine was cold, left sitting overnight. If an air intake sensor is bad, could it make the car idle low, stall when the engine is cold, and decrease fuel performance?
It's a cheap part, as far as I can tell - and even I could do it
Far difference from the mechanic quoting me $2000 for a new catalytic converter, since he was "sure that was the problem"... Despite no loss in power...

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Ok - so a quick update. Had my friend use his pro scanning tool and it came back with some odd air intake temp sensor readings. Sensor #2 read 54 degrees (correct), but sensor #1 read 200+ - this is when the vehicle/engine was cold, left sitting overnight. If an air intake sensor is bad, could it make the car idle low, stall when the engine is cold, and decrease fuel performance?
It's a cheap part, as far as I can tell - and even I could do it
Far difference from the mechanic quoting me $2000 for a new catalytic converter, since he was "sure that was the problem"... Despite no loss in power...